The theoretical maximum speed of all kinds of things is constantly going up; cars, trains, planes, boats... But in practice, we don't always get anywhere near those speeds. A good example of this is how, despite a century of technological progress, vehicles in London's city center are moving at average speeds comparable to the horse drawn carriages of yore (which also caused traffic jams), or about the same as a running chicken...

And that 10 MPH average is after a 1.5 MPH bump caused by the implementation of the congestion charge in 2003 (which is a 15% improvement -- nothing to sneeze at).